Magnetically enhanced sputter source

ABSTRACT

A circular magnetron sputter source employs a central anode surrounded by an annular cathode of generally inverted conical configuration. The anode structure forms a composite anode and inner magnetic pole piece which is operated at or near ground potential. An outer magnetic pole piece concentrically surrounds the annular cathode. This outer pole piece is electrically isolated from the cathode, and held at or near ground potential. The cathode is operated at a potential of several hundred volts negative with respect to ground. The cathode is held in place by a novel retainer means and has a novel cross section configuration. Magnetic flux is provided either by permanent magnets or by electromagnets or by a combination of both located externally of the vacuum chamber in which the sputter source is operated. Magnetic field lines pass from the outer pole piece and through the cathode, exiting the sputter surface near the outer edge of the cathode. These magnetic field lines form an arch over the cathode by passing directly to the central anode without reentering the uneroded sputter surface of the cathode. In a circular geometry, these arching magnetic field lines form an endless tunnel for confining a glow discharge. The tunnel thus formed is a novel and modified magnetic tunnel in which electrons are reflected electrostatically from the cathode surface near the outer edge of the cathode and reflected by magnetic mirroring near the inner edge. Use of this modified magnetic tunnel leads to improved electrical impedance characteristics of the glow discharge and to improved uniformity in cathode erosion.

This application is a division of application Ser. No. 150,532, filed05/16/80 now U.S. Pat. No. 4,457,825.

FIELD OF THE INVENTION

This invention is in the field of vaccum sputter coating apparatus andparticularly relates to a magnetron sputter source for such apparatus.

BACKGROUND OF THE INVENTION

Vacuum deposition of coatings is in widespread use today, and appears tobe of growing importance in the future. Cathode sputtering induced byglow discharges is emerging as one of the more important processes foreffecting such depositions. Much of the recent work relates to variousmagnetron geometries in which enhanced sputtering rates and operation atlower pressures are achieved through judicious use of magnetic fields.An extensive literature has developed and many patents have issued overthe past decade. A particularly informative and reasonably currentsummary is contained in the book "Thin Film Processes" edited by John L.Vossen and Werner Kern, published by Academic Press, New York, 1978.Particularly relevant chapters are: Chapter II-1, "Glow DischargeSputter Deposition", by J. L. Vossen and J. J. Cuomo; Chapter II-2,"Cylindrical Magnetron Sputtering", by John A. Thornton and Alan S.Penfold; Chapter II-3, "The Sputter and S-Gun Magnetrons", by David B.Fraser; and Chapter II-4, "Planar Magnetron Sputtering" by Robert K.Waits.

In order for the intensity of glow discharges to be enhanced through theapplication of magnetic fields, it is necessary that electrodegeometries, magnetic field intensities, and magnetic field geometries beselected in such a way as to produce electron traps. In most cases,crossed electric and magnetic fields give rise to electron driftcurrents which close on themselves. In the case of cylindricalmagnetrons, for example, radial electron traps can be formed withessentially uniform magnetic fields parallel to the axes of the cathodeand anode cylinders. By providing the cathode with electron reflectingsurfaces at its ends, loss of electrons from the discharge through axialdrift can be reduced, thus further enhancing the discharge intensity,and making operation at lower gas pressures possible. (See, for example,above-referenced Chapter II-2, "Cylindrical Magnetron Sputtering", byJohn A. Thornton and Alan S. Penfold, especially pp. 77-88.)

In many of the magnetrons employed commercially for sputter deposition,electron trapping is accomplished by shaping the magnetic field relativeto the shape of the sputter target (cathode). In particular, most planarmagnetrons employ a magnetic field which loops through the planarcathode surface and which forms a tunnel-shaped magnetic field whichcloses on itself. (See, for example, above-referenced Chapter II-4,"Planar Magnetron Sputtering", by Robert K. Waits, especially page 132.)Under normal operating conditions, the glow discharge is largelyconfined to this magnetic tunnel.

Magnetic tunnels are also employed with non-planar magnetronconfigurations. An example of a hollow cathode cylindrical magnetronemploying a single magnetic tunnel is shown in FIG. 4, p. 118 ofabove-referenced Chapter II-3, "The Sputter and S-Gun Magnetrons", byDavid B. Fraser. In addition, examples of cylindrical magnetronsemploying multiple magnetic tunnels are shown in FIG. 3., p. 78 ofabove-referenced Chapter II-2.

Another circular magnetron sputter source in commercial use employs acathode (target) of a generally inverted conical configurationsurrounding an axially symmetric central anode. An example of such asputter source may be found described in more detail in U.S. Pat. No.4,100,055, issued July 11, 1978 to Robert M. Rainey and entitled "TargetProfile for Sputtering Apparatus" and assigned to the assignee of thepresent invention. Such a sputter source is also commercially availablefrom and manufactured by Varian Associates, Inc. under the trademark"S-Gun". This type of sputter source is also described, for example, inabove-referenced Chapter II-3, especially FIG. 1, p. 116 and FIG. 3, p.117. In particular, FIG. 3, p. 117 shows schematically the magneticfield looping through the conical cathode (target) surface to form amagnetic tunnel which confines the glow discharge.

In prior art magnetic tunnels, the energetic electrons which sustain theglow discharge would need to cross magnetic field lines to escape fromthe magnetic tunnel, which they are unable to do if the magnetic fieldintensities are great enough. Also, those electrons which have beencaptured into the discharge are energetically incapable of reaching thecathode. Thus, even though these electrons may follow magnetic fieldlines toward the cathode surface, they will be electrostaticallyreflected from the cathode surface back into the discharge.

If the magnetic field intensity falls off with distance from the cathodesurface, as it does in most prior art magnetic tunnels, "magneticmirroring" can also contribute to electron reflection. The main effectof such magnetic mirroring is, on average, to move the region ofelectron reflection a bit further from the cathode surface. This effectis incidental rather than crucial to the magnetic tunnel's role inreflecting electrons in order to contain the glow discharge. In anyevent, those electrons which would otherwise escape through the magneticmirror will be reflected electrostatically back into the discharge. Itis therefore both convenient and proper to refer to the electronreflection in the prior art simply as "electrostatic", even though somemagnetic mirroring may be occurring.

Discharge intensity tends to be a maximum in the center of a magnetictunnel, where the magnetic field lines are generally parallel to thecathode surface, and falls off rapidly as the sides of the magnetictunnel are approached. Localized cathode (target) erosion ratescorrespond generally with the immediately adjacent intensity of the glowdischarge, thus leading to nonuniform erosion of the cathode surface.Examples of nonuniform erosion of an S-Gun cathode are shown in FIG. 3of above-referenced U.S. Pat. No. 4,100,055 to Rainey, and examples inthe case of planar magnetron cathodes are shown in FIG. 5, p. 141 ofabove-referenced Chapter II-4.

One consequence of nonuniform cathode erosion is that there isless-than-maximum utilization of target material. Another consequence ofnonuniform cathode erosion is that changes may occur in the distributionpattern of sputtered material leaving the cathode surface. Additionally,the glow discharge tends to move downward in the magnetic tunnel tomaintain close proximity to the cathode surface as the cathode surfaceerodes away. This movement when coupled with nonuniform cathode erosiontends to concentrate the discharge even more sharply, leading to stillgreater nonuniformity of cathode erosion. Furthermore, such nonuniformcathode erosion restricts the area of emission of sputtered atoms to arelatively narrow band on the cathode surface. This in turn restrictsthe range of direction of sputtered atoms arriving at the substrate tobe coated, thus affecting such film properties as uniformity and stepcoverage, both of which are of particular importance in metalization ofsemiconductor wafers, for example. Also, the deposition rate from adeeply eroded cathode may be reduced because of geometrical shieldingeffects. In addition, nonuniform cathode erosion is attended bycorrespondingly nonuniform cathode heating, which contributes adverselyboth to cathode cooling problems and to thermal stressing of thecathode.

Using prior art magnetic tunnels, a further consequence of the movementof the glow discharge with erosion of the cathode surface is that thedischarge generally moves into a region of greater magnetic fieldintensity. This results in a lowering of the discharge impedance, whichrequires lower operating voltage, higher dishcarge current, and higherdischarge power to maintain a fixed deposition rate (seeabove-referenced Chapter II-2, pp. 94-98; also see above-referencedChapter II-3, pp. 117-121). An indication of the severity of thisproblem in some applications is conveyed by U.S. Pat. No. 4,166,783issued Sept. 4, 1979 to Frederick T. Turner and entitled "DepositionRate Regulation by Computer Control of Sputtering Systems" and assignedto the assignee of the present invention.

Accordingly, it is an object of the invention to provide a glowdischarge sputter source in which input power can be maintained constantthroughout cathode (target) life for constant deposition rate.

Another object of the invention is to provide a sputter source whichoperates with higher sputtering efficiency, whereby power consumptionand power supply size are reduced.

Another object of the invention is to provide a sputter source in whichthe electrical impedance of the glow discharge remains substantiallyconstant throughout cathode (target) life, whereby the problems ofsupplying and controlling power are reduced.

A further object of the invention is to increase the utilization oftarget material, thereby increasing target life.

Yet another object of the invention is to maintain a more uniformdistribution pattern of sputtered material leaving the cathode surfaceover the useful life of the cathode.

A further object of the invention is to increase the width of the bandfrom which sputtered atoms are emitted from the cathode, wherebycoatings having improved properties may be obtained.

A still further object of the invention is to ease the cathode coolingproblem, thereby allowing operation at higher powers and atcorrespondingly greater sputter deposition rates.

Yet another object of the invention is to reduce thermal stressing ofthe cathode, whereby fracture, localized melting, and the like areavoided.

A further object of the invention is to provide an improved means forholding the cathode in place, whereby brittle and weak cathodes may beretained without breakage caused by the holding means.

A still further object is to reduce the amount of unused target materialin the cathode.

SUMMARY OF THE INVENTION

In prior art magnetic tunnels as applied to sputter sources, themagnetic field loops through the cathode surface to confine the glowdischarge within the magnetic tunnel. As discussed earlier, confinementof the discharge occurs because electrons are reflectedelectrostatically from the cathode surface back into the discharge. Suchelectrostatic reflection takes place on "both sides" of the magnetictunnel.

In the preferred embodiment of the present invention, a modifiedmagnetic tunnel is employed in which only a "first side" of the magnetictunnel passes through the cathode surface. Confinement of the glowdischarge on the "second side" is provided by a "magnetic mirror".Electron reflection thus occurs electrostatically from the cathodesurface on the first side of the magnetic tunnel, whereas electrons arereflected by magnetic mirroring on the second side.

One feature of magnetic mirrors which is of particular importance in thepresent invention is that the mirrors are "soft" in the sense that thepoint of reflection is not a well defined physical surface, but dependson the ratio of parallel to perpendicular electron velocities relativeto the direction of the magnetic field at some interior point. This"softness" of the magnetic mirrors is used to advantage in the presentinvention to produce a widening and a spreading out of the dischargetoward the "magnetic mirror side" of the discharge. This contributes toa broader, less sharply concentrated cathode erosion pattern.

One consequence of using one or more magnetic mirrors is that it becomespossible to design magnetic tunnels which are much flatter, that is,much less sharply arching, than the prior art magnetic tunnels. Thiscontributes to increased uniformity of cathode erosion, and can leadalso to glow discharges whose electrical impedance at a fixed powerlevel and operating pressure remains more constant as the cathode iseroded away. In certain applications this means that a much simplercontrol system can be used to obtain the desired deposition rate thanhas been possible heretofore with sputter sources employing prior artmagnetic tunnels. It also means that less flexibility is required of thepower supply, which leads to reductions in size and cost as comparedwith the power supplies required for prior art sputter sources employingmagnetic tunnels.

Thus it is that the use of a magnetic tunnel employing one or moremagentic mirrors allows many of the objects of the present invention tobe realized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial section of a sputter source incorporating thepresent invention in the preferred embodiment.

FIG. 2 is a section of a prior art sputter source.

FIG. 3 is a fragmentary section of the sputter source of FIG. 1 showingnew and end-of-useful-life cathode profiles and showing also magneticfield lines and data.

FIG. 4 is a fragmentary section of the prior art sputter source of FIG.2 showing new and end-of-useful-life cathode profiles and showing alsomagnetic field lines and data.

FIGS. 5a and 5b show normalized deposition rates as functions of cathodelife in kilowatt-hours for the sputter sources of FIGS. 1 and 2respectively.

FIGS. 6a and 6b show voltage-current curves with argon pressure as aparameter for the sputter sources of FIGS. 1 and 2 respectively.

FIG. 7a shows schematically magnetic field lines converging to form amagnetic mirror, together with representative electron trajectories.

FIG. 7b shows schematically the relative magnitude of the magnetic fieldintensity along a representative magnetic field line of FIG. 7a.

FIG. 8 is a fragmentary section of the sputter source of FIG. 1 showingin greater detail the cathode retaining means.

DETAILED DESCRIPTION

The preferred embodiment of the present invention is shown in FIG. 1wherein the sputter coating source 1 is of generally circularconfiguration. A circular central anode 10 is surrounded by a circularcathode ring 12 having a front sputter surface 13 of generally invertedconical configuration from which material is to be sputtered. The ringmember 12 is at negative potential relative to anode 10 during operationof the sputter coating source and thus is aptly termed a cathode. Ringmember 12 also forms a target for bombardment by ions from the glowdischarge and thus is also referred to in the art as a sputter target.Accordingly, ring 12 is referred to in various places in the descriptionand claims alternatively as a cathode or a sputter target. The detailsof the cross sectional shape of the cathode (sputter target) 12 will bedescribed hereinafter in respect to FIG. 8.

Anode 10 serves both as an electrical field forming electrode and as oneend of the magnetic field-forming circuit. More specifically, anode 10comprises a magnetic pole piece 15, and in order to facilitate insertionand removal of the cathode (as will be hereinafter described in detail),pole piece 15 preferably includes a removable annular ring portion 16.Also, a removable thin anode surface sheet in the form of inverted cup17 is held in place by screws 18 (one shown). Anode surface sheet 17 canbe magnetic or nonmagnetic material, but if nonmagnetic, it should besufficiently thin to preserve the desired magnetic field strength at theanode surface. An annular member 20, made of nonmagnetic material, isattached by means of bolts 21 to pole piece 15. An inner O-ring groove22 allows a vacuum tight seal to be made between annular member 20 andpole piece 15. Annular member 20 also contains outer O-ring groove 23for sealing to the lower side of an electrical insulator ring 24 toinsulate anode 10 from cathode 12. Anode 10 including pole piece 15 iscooled by passing coolant through a water channel 26 via coaxialconduits 27 and 28. An inverted cup-shaped magnetic member 30 is securedto pole piece 15 by means of bolts 31 (one shown). An O-ring groove 32is provided in pole piece 15 to prevent coolant leakage between polepiece 15 and magnetic member 30. Annular magnets 33 provide the magneticfield for the magnetically enhanced sputter source. Because magnets 33are located outside of the vacuum chamber, they need not be made ofvacuum-compatible materials. Thus, for example, magnets 33 may be madeof a barium ferrite permanent magnet material such as Indox 5.Alternatively, an annular electromagnet (not shown) may be used incombination with permanent magnets 33 to provide an electricallycontrollable portion of the magnetic field. Such electrical control ofthe magnetic field can be used to adjust the electrical impedance of theglow discharge, whereby, for example, changes in discharge impedancewith cathode erosion can be compensated. In addition, a temporaryincrease in magnetic field can be advantageously used to triggerdischarge initiation. Magnets 33 are placed on a magnetic base plate 34,onto which they are held by magnetic attraction. Adequate centering ofmagnets 33 is achieved through the use of a nonmagnetic cylinder 35secured to flange 36, which is secured in turn to base plate 34 byscrews 37 (one shown). A magnetic ring 38 is placed between magneticmember 30 and upper magnet 33. Magnetic members 30 and 38 and magnets 33are held together by magnetic attraction.

Cathode 12 is secured, by novel means which are detailed later, to anonmagnetic annular base member 40. Cathode 12 is also surrounded bynonmagnetic water jacket 41. Cathode 12 and water jacket 41 are sodimensioned that room temperature clearance between these members islarge enough to allow easy installation and removal, yet small enough toprovide adequate thermal contact for cathode cooling when the cathodeexpands upon heating during normal operation. Water jacket 41 is securedmechanically to base member 40 by means of a nonmagnetic ring member 42held by screws 43 (one shown). Water jacket 41 has internal waterchannels 45 through which coolant, preferably water, is circulated viaconduits 50 (one shown). Conduits 50 are brazed in sleeves 51 which arebrazed in base member 40 to provide vacuum-tight sealing betweenconduits 50 and base member 40. Conduits 50 also comprise conventionaldetachable compression fittings 52 and 53 plus a bellows member 54,which is employed to reduce mechanical stress on the vacuum-tightsealing of conduits 50 to base member 40. Direct cooling of base member40 is provided by water channel 56 through which coolant is circulatedvia conduits 57 (one shown). This cooling is of particular importance inpreserving the vacuum integrity of the O-ring in an O-ring sealinggroove 58 for sealing the upper side of anode insulator 24. Base member40 also contains an O-ring sealing groove 55 for sealing to the lowerside of an electrical insulator ring 59 for the cathode. Finally, basemember 40 has secured to it, by tack welding for example, a cathoderetainer ring 60 which is shown in greater detail in FIG. 8. A shieldring 61 has an outer lip portion which is sandwiched between the top ofretainer ring 60 and cathode 12. The purpose of shield 61 is to reduceundesired coating of anode insulator 24 during sputter source operation.Retainer ring 60 contains a plurality of threaded holes, and shield 61contains a plurality of corresponding clearance holes which are broughtinto registry during assembly.

As shown in greater detail in FIG. 8, cathode 12 contains an inner rimportion 62 including an annular groove having angled wall 63 which makesan acute angle of about 60° with the bottom or back surface 64 ofcathode 12. The threaded holes in retaining ring 60 engage threadedmembers 65, which may be dog-point set screws, for example, or,alternatively, special screws incorporating spring-loaded ball plungers.Tightening threaded members 65 against angled wall 63 by inserting atool through holes 66 in shield ring 61 provides positive retention ofcathode 12 upon normal installation at room temperature. The number ofthreaded members 65 employed is preferably three. When cathode 12expands upon heating during normal operation, it may expand away fromthreaded members 65. However, acutely angled wall 63 in cooperation withthreaded members 65 serve to prevent cathode 12 from falling asignificant distance away from base member 40 in case the sputter sourceis operated in an inverted position, for example. Moreover, thermalexpansion of cathode 12 during normal operation tends to hold itsecurely in water jacket 41. Replacement of cathode 12 is accomplishedby removing anode surface sheet 17 and annular ring portion 16 from polepiece 15, and then unscrewing threaded members 65 enough to release thecathode, which in turn releases shield ring 61 which is simply held inplace by the presence of cathode 12.

As shown in detail in FIG. 8, cathode 12 has an outer surface or rimformed by a lower portion 67 and an upper surface portion 68 of largerdiameter than the lower portion 67. The upper and lower portions arepreferably joined by a sloping intermediate portion 69. The relativeshaping and positioning of inner pole piece 15 and the hereinafterdescribed outer pole piece 72 cooperates with the above-describedshaping of the cathode 12 to obtain the desired magnetic field shapewith respect to cathode 12. These relationships and resulting erosionpattern have also been found to permit the direct cooling of cathode 12to be limited to the area of lower wall portion 67, as distinguishedfrom having the outer surface of the cathode extend straight down fromthe large diameter outer portion 68. Thus the cathode shape in FIGS. 1and 8 results in a smaller overall diameter for the sputter source 1with attendant savings in cost of materials and space occupied by thesource. The above relationships also result in the thick inner rim 62 oncathode 12 which permits use of the novel angled wall 63 and threadedmembers 65 for holding the cathode in place.

A housing 70 for the anode-cathode assembly comprises a lower ringmember 71 and an outer magnetic pole piece ring 72 joined together invacuum-tight fashion by a cylindrical wall member 73. Members 71 and 73are made of ferromagnetic material, such as cold rolled steel, toprovide portions of the required magnetic path to pole piece 72. Lowerring member 71 contains O-ring sealing groove 74 to facilitatedemountable and vacuum-tight installation of the sputter source of FIG.1 in the wall of the vacuum chamber (not shown) so that the sputtersource projects from the chamber wall into the chamber. Pole piece 72also contains O-ring sealing groove 77 to allow a vacuum-tight seal tobe made to the upper side of cathode insulator 59. A concentric pair ofcylindrical flash-over insulators 78 and 79 is provided to preventarcing to wall member 73 during sputter source operation. Removablyattached (attachment means not shown) to outer pole piece 72 arenonmagnetic ground shield members 80 and 81, with water coolednonmagnetic member 82 positioned between the two ground shields andcooled via water flowing through attached conduit 83. Ground shield 80serves particularly to reduce undesired coating of cathode insulator 59during sputter source operation.

The overall assembly of the sputter source of FIG. 1 is held together bymeans of clamping ring member 90. Bolts (not shown) draw clamping ringmember 90 toward lower ring member 71 by passing through hole 91 andengaging threads in hole 92. In so doing, clamping ring member 90 forcesbase plate 34 upward, thereby effecting vacuum-tight seals bycompressing O-rings in O-ring sealing grooves 23 and 58 on the lower andupper sides respectively of anode insulator 24, and also by compressingO-rings in O-ring sealing grooves 55 and 77 on the lower and upper sidesrespectively of cathode insulator 59.

After the sputter source is installed in the vacuum chamber and thechamber is evacuated, atmospheric pressure acts to compress thejust-mentioned O-rings even further, thereby contributing positively tothe vacuum integrity of the O-ring seals. This additional compression ofthe O-rings leads to an upward movement of base plate 34 and,correspondingly, to a reduction in tension of the bolts (not shown)which draw clamping ring member 90 toward lower ring member 71. Suchreduction in bolt tension may allow clamping ring member 90 to rattlearound loosely, thereby motivating an operator to retighten the bolts.This, if done, could lead to overstressing of the bolts and/or clampingring member 90 when the vacuum system is let back up to atmosphericpressure. This problem is avoided through the use of special boltsincorporating spring-loaded ball plungers (not shown) screwed intothreaded hole 93 in clamping ring member 90. The spring-loaded plungerspress against base plate 34, thereby maintaining tension on the bolts(not shown) engaging threaded holes 92 after base plate 34 has movedforward upon vacuum system evacuation.

As will be discussed in greater detail, the objects of the presentinvention are realized by employing a modified magnetic tunnel in whichone side of the magnetic tunnel is formed by a magnetic mirror. Designof the magnetic circuit overall, including particularly the geometriesof the center anode pole piece 15 and outer pole piece 72, has led tothe pattern of magnetic field lines 95 shown in FIG. 3. It should benoted that the arching magnetic field lines above uneroded cathodesputter surface 13 do not loop through the cathode surface, as they doin many prior art sputter sources. Rather, those magnetic field lineswhich do pass through cathode sputter surface 13 go directly to anode 10rather than passing a second time through cathode sputter surface 13. Itwill be established subsequently that electron reflection from anode 10back into the glow discharge occurs due to magnetic mirroring with thisparticular magnetic field configuration.

In typical operation, the chamber in which the sputter source is mountedis evacuated to a pressure on the order of 10⁻⁶ Torr. The chamber isthen back-filled with a sputter gas, which is typically argon, to apressure in the range of 0.1 to 100 mTorr. Ground shields 80 and 81 andanode 10 are normally held at ground potential (although anode 10 may bebiased slightly above ground potential in some applications), and apotential in the range -350 volts to -1,000 volts with respect to groundis applied to cathode 12, depending on such details as anode and cathodegeometry, magnetic field intensities, cathode material, sputter gasspecies, sputter gas pressure, and desired discharge current. By way ofexample, electrical connection to cathode 12 may be made by a connectionto cooling conduit 50, and electrical connection to anode 10 may be madeby a connection to cooling conduit 27.

It may be noted that inner magnetic pole 15 is part of anode 10, whichis operated at or near ground potential. Outer magnetic pole piece 72 iselectrically isolated from cathode 12, and is held at or near groundpotential also. Because cathode 12 is operated at a potential of severalhundred volts negative with respect to ground, ion bombardment of thepole pieces with attendant sputtering cannot occur. Thus the possibilityof contamination of sputter-deposited coatings due to pole piecesputtering is avoided.

Shown in FIG. 2 is a prior art sputter source of cylindrically symmetricgeometry manufactured and sold by Varian Associates, Inc. under thetrademark "S-Gun". The S-gun sputter source is described inabove-referenced Chapter II-3, especially FIG. 1, page 116 and FIG. 3,page 117. Descriptions in greater detail are provided by aforementionedU.S. Pat. No. 4,100,055 to Rainey, and also by U.S. Pat. No. 4,060,470to Peter J. Clarke.

In FIG. 2, a central anode 110 is made of nonmagnetic material, such ascopper, and is surrounded by an annular cathode 112. Anode 110 ismounted on anode post 115, which is nonmagnetic and is preferably madeof copper. Anode post 115 has internal cooling cavity 120 through whichwater circulates via conduits 121. Anode post 115 is mounted, eitherconductively or insulatively, on nonmagnetic base plate 129 by means offlange 123.

Cathode 112 has a sputter surface 113 of generally inverted conicalconfiguration. Cathode 112 is mounted on lower magnetic pole piece 142,and is surrounded by nonmagnetic water jacket 144. Clamping ring 165 isoptionally provided to secure cathode 112 to pole piece 142. Cathode 112and water jacket 144 are so dimensioned that room temperature clearancebetween them is sufficient to allow easy installation and removal of thecathode, yet small enough to provide adequate thermal contact forcathode cooling when the cathode expands upon being heated during normaloperation. Water jacket 144 has internal water channel 145 through whichcoolant, preferably water, is circulated via conduits 150. Conduits 150are secured to base plate 129 by means of flanges 155. Electricalisolation between base plate 129 and conduits 150 is achieved by makingconduits 150 of electrically nonconducting materials. Additional supportmeans (not shown) are employed to ensure that the desired spacingbetween lower pole piece 142 and base plate 129 is maintained.

The main magnetic field for this prior art magnetically enhanced sputtersource is provided by a first plurality of bar magnets 128 (made, forexample, of a vacuum-compatible permanent magnet material such as Alnico8) arrayed annularly between lower magnetic pole piece 142 and an uppermagnetic pole piece 172. A second plurality of bar magnets 128' isarrayed annularly above upper pole piece 172 and in magnetic opposition(or in bucking magnetic field arrangement) to the main magnetic field.The principal purpose of the bucking magnetic field arrangement is tosuppress stray glow discharges in the region above the upper pole piece172. A nonmagnetic cylinder 130 defines the outer limits for accuratelylocating the magnets 128 and 128' with respect to pole pieces 142 and172, and nonmagnetic ring 176 serves to further suppress stray glowdischarges above pole piece 172. The resulting magnetic field lines 195are shown best in FIG. 4. Of particular interest are those magneticfield lines which arch above and through the sputter surface 113 ofcathode 112 to form a magnetic tunnel for confining the glow discharge.

Further, surrounding cathode 112, but electrically isolated therefrom,is a generally cylindrical and nonmagnetic outer housing 170 comprisingouter ground shield member 173 conductively attached to base plate 129,and separable inner ground shield member 180.

In general terms, operation of this prior art sputter source of FIG. 2is similar to that described above for the sputter source of FIG. 1. Thesignificant differences between the prior art sputter source of FIG. 2and the sputter source of FIG. 1, which is the preferred embodiment ofthe present invention, will be elucidated below.

From the descriptions thus far, the preferred embodiment of FIG. 1 andthe prior art sputter source of FIG. 2 are superficially very similar.One incidental difference is physical size, with, for example, the outerdiameter of cathode 12 being approximately 7.00 inches whereas the outerdiameter of cathode 112 is approximately 5.15 inches. The significantdifferences, however, lie in the magnetic field configurations and themagnetic circuits employed to achieve them. The magnetic fieldconfigurations in the vicinity of the cathodes are shown in detail inthe fragmentary cross sectional views of FIG. 3 for the preferredembodiment and FIG. 4 for the prior art. In these figures, the new oruneroded cathode sputter surfaces are indicated by 13 and 113respectively, while the end-of-useful-life cathode sputter surfaceprofiles are indicated by 13' and 113'. These profiles were obtainedwith aluminum cathodes after operation for 400 kilowatt-hours and 148kilowatt-hours, respectively. Measured magnetic field data are alsodisplayed in FIGS. 3 and 4. In encircled data points 96 in FIG. 3 and196 in FIG. 4, for example, the local direction of the magnetic field isindicated by the short, heavy line segment, and the magnitude of themagnetic field in gauss at the midpoint of the line segment is indicatedby the adjacent number (180 gauss in the instance of encircled datapoint 96, and 103 gauss in the case of encircled data point 196).Selected magnetic field lines 95 and 195 have been constructed ingeneral conformance with the measured magnetic field data points.

In the case of the prior art sputter source of FIG. 4, magnetic fieldlines 195 which arch through uneroded cathode sputter surface 113 (thatis, magnetic field lines which extend from a first region of the cathodesputter surface and return to a second region thereof) form arched fieldlines along which electrons tend to travel. As the electrons approachthe cathode surface they are mirrored or reflected back and are thusretained in a so-called tunnel formed by magnetic field lines which ateach end intersect a surface at cathode potential. Such a tunnel can beaptly named a magnetic-electrostatic tunnel. Since the cathode and polepieces are annular, the magnetic-electrostatic tunnel is a closed looptunnel and thus retains the electrons which tend to precess in adirection into the paper and would escape from an open ended tunnel.Provided the magnetic field intensities are great enough, suchmagnetic-electrostatic tunnels serve to confine and magnetically enhanceglow discharges.

In the case of the preferred embodiment of FIG. 3, however, the magneticfield lines which arch above the cathode and which pass hrough unerodedcathode sputter surface 13 do so only once, rather than twice as in theprior art case of FIG. 4. One class of magnetic field lines emanatesfrom outer pole piece 72 and exits cathode sputter surface 13 near theouter diameter of cathode 12; these magnetic field lines do not reentercathode 12, but rather form an arch from cathode sputter surface 13 tothe anode. In normal circumstances, magnetic field lines passing throughan electrode held at a positive potential with respect to the cathodeprovide a ready conduit by which electrons can escape from thedischarge. Such magnetic field lines would therefore not be expected tobe effective in confining and magnetically enhancing glow discharges. Inthis particular case, however, care has been taken to ensure that themagnetic field intensity increases sufficiently that an adequatefraction of the electrons is reflected by magnetic mirroring, as will bediscussed later. A modified electron capture tunnel is thus realizedwhich is effective in confining and magnetically enchancing glowdischarges.

Under conditions of normal operation, the glow discharge is confined bythe modified electron capture tunnel above the sputter surface of thecathode. The negative glow region of the discharge, which is where mostof the ions are produced by electron-gas atom or electron-gas moleculecollisions, is separated from the sputter surface of the cathode by thecathode dark space. The thickness of the cathode dark space is dependenton several parameters, including anode and cathode geometries, magneticfield intensities, cathode material, sputter gas species and pressure,and discharge current. In representative cases, however, the cathodedark space thickness is approximately one millimeter, and the negativeglow region of the discharge extends to several millimeters above thesputter surface of the cathode. Beyond these rather general statements,it is not a simple matter to provide a more complete description of theglow discharge on theoretical grounds.

We can, however, make use of cathode erosion patterns to draw certaininferences about the glow discharges. This is so because localizedcathode erosion rates correspond generally with the immediately adjacentintensity of the discharge. On this basis it would appear that thesituation with prior art sputter sources of FIG. 4 goes about asfollows. When cathode 112 is new and the sputter surface is defined by113, the glow discharge is confined by a relatively wide magnetictunnel, whereby the discharge extends over most of the cathode sputtersurface 113. Even so, discharge intensity will be greater near thecenter of the magnetic tunnel than near the sides, leading tocorrespondingly more rapid cathode erosion near the magnetic tunnelcenter. As cathode erosion proceeds, the discharge is confined bymagnetic tunnels of progressively smaller width and larger magneticfield intensities. In addition, the centers of the magnetic tunnels moveoutward in radial position. By the time end-of-useful-life cathodeprofile 113' is reached, most of the discharge is concentrated in arelatively narrow ring near the outer edge of cathode 112, and themagnetic field intensity averaged over the discharge may have increasedby, perhaps, 100%, or even more.

In the case of the preferred embodiment of FIG. 3, the magnetic tunnelswhich confine the glow discharge are generally much flatter, that is,much less sharply arching, than the magnetic tunnels of FIG. 4. Ascathode erosion proceeds, the center of the magnetic tunnel of FIG. 3moves radially outward, but less rapidly than the center of the magnetictunnel of FIG. 4. In addition, the magnetic field intensity averagedover the discharge changes much less rapidly with cathode sputtersurface erosion in the case of FIG. 3 than in the case of FIG. 4, theincrease by end-of-useful-life being, perhaps, in the 30% to 40% range.

Several consequences result from the significant differences in magneticfield configurations of FIGS. 3 and 4. One of the more importantconsequences is that the electrical impedance of the glow discharge ishigher and changes much less over cathode life in the case of FIG. 3.This in turn means that the operating voltage is higher, and that thevoltage and current change correspondingly less at a given dischargepower level. At this higher operating voltage, sputter yield increasesnearly linearly with voltage. This means that the sputter depositionrate can be held essentially constant throughout cathode life by holdingthe input power constant when the relatively small changes in dischargeimpedance do occur. Experimental support for the above statement isprovided in FIGS. 5a and 5b in which normalized deposition rates areplotted against cathode life in kilowatt-hours. As shown in FIG. 5a forthe preferred embodiment of the present invention, the variation in thenormalized deposition rate is less than the ±4% measurement uncertaintyover cathode life extending out to 375 kilowatt-hours. By way ofcomparison, the case for the prior art sputter source of FIGS. 2 and 4is shown in FIG. 5b, in which the normalized deposition rate has fallenby more than 40% after 140 kilowatt-hours of cathode life. The mainreason for this decline in normalized deposition rate is that thedischarge impedance has fallen, leading to lower voltage of operation,and, correspondingly, to even lower sputter yield. A second reason isthat geometrical shielding by the sputter surface of the cathode itselfreduces the deposition rate as the end-of-useful-life cathode sputtersurface profile 113' is approached.

In some applications the variation of normalized deposition rate withcathode life leads to serious problems in deposition rate control. Oneeffort to control deposition rate in the face of the variation as shownin FIG. 5b is disclosed in earlier-referenced U.S. Pat. No. 4,166,738 toTurner and entitled "Deposition Rate Regulation by Computer Control ofSputtering Systems". With the substantially constant normalizeddeposition rate shown in FIG. 5a, a much simpler control system can beused to obtain the desired deposition rate.

Another consequence of the declining normalized deposition rate of FIG.5b is that input power to the sputter source must be increased if aconstant deposition rate is to be maintained. If, for example, thenormalized deposition rate has fallen by 40%, it is necessary toincrease input power by 67% to obtain the beginning-of-life depositionrate. Thus, more power is consumed; correspondingly, sputter sourcecooling problems are aggravated; and the power supply must be larger,more flexible, and more expensive than would otherwise be the case. Allof these problems are eased substantially with the preferred embodimentof the present invention because of the essentially constant normalizeddeposition rate shown in FIG. 5a.

Another important consequence of the novel magnetic field configurationof FIG. 3 is that a significantly larger fraction of the cathodematerial can be utilized than with the prior art sputter source havingthe magnetic field configuration of FIG. 4. In the case of cathode 12made of aluminum, the weight of cathode 12 when new, that is, withuneroded cathode sputter surface 13, is 900 grams. After 400kilowatt-hours of typical operation, end-of-useful-life cathode sputtersurface profile 13' has been reached, with a weight loss of 560 grams.Thus, by-end-of-useful-life 62% of the cathode material has beenutilized. By contrast, the new weight of prior art cathode 112 is 285grams, and the end-of-useful-life weight loss is 151 grams after 148kilowatt-hours, which corresponds to 53% material utilization, Thus thepreferred embodiment of the present invention has 17% greater materialutilization than the prior art sputter source of FIGS. 2 and 4.

Yet another important consequence of the novel magnetic fieldconfiguration of FIG. 3 is that the voltage across the glow discharge issignificantly higher than with the prior art sputter source having theconventional magnetic-electrostatic field configuration of FIG. 4. Thispoint is illustrated in FIGS. 6a and 6b by the voltage-current curvestaken at various argon pressures. FIG. 6a applies to the preferredembodiment of the present invention as disclosed in FIGS. 1 and 3, whileFIG. 6b applies to the prior art sputter source described in FIGS. 2 and4. For example, with argon as the sputter gas at a pressure of 10 mTorrand at an operating power level of 4.0 kilowatts, the magnitudes ofvoltage and current from FIG. 5a are about 605 volts and 6.6 amperes,while the voltage and current from FIG. 6b are about 410 volts and 9.8amperes. The new sputter source thus operates at more than 47% highervoltage than the prior art sputter source in the example given. As thecathodes erode, this difference will become even greater, with theoperating voltage of the new sputter source changing by a relativelysmall amount while the operating voltage of the prior art sputter sourceis declining to a significantly greater extent (see the earlierdiscussion relative to FIGS. 5a and 5b). The higher operating voltage ofthe new sputter source means that it operates at a higher sputter yield,thereby reducing the power required to achieve the desired depositionrate, thus contributing to reductions in costs for power itself, forcooling, and for power supplies.

Further evidence for higher sputtering efficiency with the new sputtersource is provided by the earlier paragraph dealing with utilization ofcathode material. It was reported there that the cathode weight loss was560 grams after 400 kilowatt-hours for the new sputter source; thiscorresponds to an average material removal rate of 1.40 grams perkilowatt-hour. For the prior art sputter source, however, the cathodeweight loss was 151 grams after 148 kilowatt-hours, which leads to anaverage material removal rate of 1.02 grams per kilowatt-hour. Thesputtering efficiency for the new sputter source is thus 37% greaterthan that of the prior art sputter source.

An additional favorable feature of the new sputter source may be notedfrom FIGS. 6a and 6b. During sputter source operation, it is frequentlydesirable to maintain constant power in the face of small changes whichmay occur in sputter gas pressure. For example, with argon as thesputter gas and with an operating power level of 4.0 kilowatts, a changein argon pressure from 4 mTorr to 10 mTorr requires in the case of thenew sputter source (FIG. 6a) that the magnitude of the applied voltagechange from 740 volts to 610 volts; the voltage change of 130 voltsdivided by the average voltage of 675 volts is 0.193. With the samepower level and argon pressure for the prior art sputter source (FIG.6b), the required voltage changes from 525 volts to 410 volts; thevoltage change of 115 volts divided by the average voltage of 462 voltsis 0.249. Thus in this example the fractional voltage change required tomaintain constant power is about 22% less for the new sputter sourcethan for the prior art sputter source. This means that the problem ofmaintaining constant power in the face of sputter gas pressure changesis correspondingly easier with the new sputter source.

It has been stated previously that the objects of the present inventionare realized in the preferred embodiment by employing a modifiedmagnetic tunnel in which one side of the magnetic tunnel is formed by amagnetic mirror. In prior art magnetic tunnels, the energetic electronswhich sustain the glow discharge are confined by magnetic field lineswhich arch or loop through the cathode sputter surface. Electrons tendto follow magnetic field lines as they move toward and away from thecathode. Those electrons which have been captured into the discharge areenergetically incapable of reaching the cathode. Thus, even though theseelectrons may follow magnetic field lines toward the cathode sputtersurface, they will be electrostatically reflected from the cathodesputter surface back into the discharge. (The incidental presence ofmagnetic mirroring in prior art magnetic tunnels was discussed brieflyin the "Background of the Invention" section of the presentapplication.)

In the preferred embodiment of the present invention, as shown in FIG.3, the side of the magnetic tunnel near the outer edge of cathode 12 isformed by magnetic field lines which, when the cathode is new, passthrough uneroded cathode sputter surface 13. These magnetic field linesdo not reenter cathode 12, but instead form an arch from unerodedcathode sputter surface 13 to anode 10. Along the outer side of themagnetic tunnel, electrons are reflected electrostatically from cathodesutter surface 13 back into the glow discharge, just as in the case forboth sides of prior art magnetic tunnels. On the inner side of themagnetic tunnel, however, electrostatic forces attract the electronstoward, rather than repelling them from, anode 10. Reflection of anadequate fraction of the electrons is achieved by employing a magneticfield configuration in which the magnetic field intensity increasessufficiently as the electrons approach anode 10. Such a magnetic fieldconfiguration is referred to as a magnetic mirror. Thus, a modifiedmagnetic tunnel is formed in which the magnetic field lines (1) causeelectrons to be reflected electrostatically, in the usual prior artfashion, near the outer edge of cathode 12, and (2) cause electrons tobe reflected by magnetic mirroring near the inner edge of cathode 12.

To better understand how magnetic mirrors function, reference may bemade to FIGS. 7a and 7b. FIG. 7a displays schematically a magneticmirror in which magnetic field lines converge from left to right. FIG.7b displays, also schematically and on the same scale, the relativemagnitude of magnetic field intensity B(z) along some representativemagnetic field line. Two representative electron trajectories are shownin FIG. 7a. υ.sub.∥ (Z) and υ.sub.⊥ (Z) are, respectively, the paralleland perpendicular components of electron velocity. In the first electrontrajectory, at z=O, υ.sub.∥ (O) is small in comparison with υ.sub.⊥ (O),and electron reflection occurs at z=z₁, while in the second electrontrajectory, υ.sub.∥ (O) is appreciable with respect to υ.sub.⊥ (O), andelectron reflection takes place at z=z₂. As will be established shortly,z₂ is greater than z₁. As an electron moves from left to right in aregion of increasing magnetic field intensity, υ.sub.⊥ (Z) increases atthe expense of υ.sub.∥ (Z). Electron reflection occurs where υ.sub.∥ (Z)goes to zero; this is the magnetic mirror point. In the absence of anelectric field acting on the electrons, conservation of energy of theelectrons requires that

    υ.sub.195 (Z.sub.r)=[υ.sub.⊥.sup.2 (O)+υ.sub.∥.sup.2 (O)].sup.1/2

where z_(r) is the value of z at which electron reflection occurs.

Also in the absence of an electric field, gyrating electrons tend toconserve their magnetic moment, that is, ##EQU1## This relationshipcombined with the conservation of energy relationship leads to thecondition for magnetic mirroring, which may be written as ##EQU2## or,alternatively, as ##EQU3## (The following references may be useful inrespect to the derivation of the condition for magnetic mirroring: JohnDavid Jackson, "Classical Electrodynamics", John Wiley and Sons, Inc.,New York, 1962, pp 419-424. Nicholas A. Krall and Alvin W. Trivelpiece,"Principles of Plasma Physics", McGraw Hill Book Company, New York,1973, pp 622-623. Francis F. Chen, "Introduction to Plasma Physics",Plenum Press, New York, 1974, pp 23-31.)

Examination of the magnetic mirror equations reveals that reflection bya magnetic mirror, unlike electrostatic reflection, is not absolute.Suppose, for example, B(z) approaches a maximum value of 2B(O). Allelectrons for which υ.sub.∥ (O) is less than υ.sub.⊥ (O) will bereflected from right to left, while all of those electrons for whichυ.sub.∥ (O) is greater than υ.sub.⊥ (O) will escape to the right. Thus,those electrodes in a glow discharge having υ.sub.∥ (O) greater thanυ.sub.⊥ (O) will be lost from the discharge. It may be useful at thispoint to define the "strength" of a magnetic mirror as the ratio of themaximum value of B(Z) to B(O). In the example just considered, thestrength of the magnetic mirror would be 2.

A second feature of magnetic mirrors revealed by examination of themagnetic mirror equations is that the mirror is "soft" in the sense thatthe region of electron reflection is not a well-defined physicalsurface, but depends on the ratio of parallel to perpendicular electronvelocities at the interior plane defined by Z=0. For example, if υ.sub.∥(O)=0.1υ.sub.⊥ (O), the mirror point will occur at the value of z atwhich B(z_(r))=1.01B(O). Similarly, if υ.sub.∥ (O)=0.5υ.sub.⊥ (O)electron reflection will occur when B(z_(r))=1.25B(O).

In the above treatment of magnetic mirrors it was assumed that noelectric fields are acting on the electrons. To the extent that electricfields are present, electron trajectories and points of reflection willbe modified.

When voltage is applied to uneroded cathode 12 of FIGS. 1 and 3 withoutsputter gas being present, a glow discharge will not normally beestablished, and the electric field distribution in the space aboveuneroded cathode sputter surface 13 can be estimated or measured bystraightforward methods. Upon establishing a glow discharge byintroducing a sputter gas, a very different electric field distributionwill come into existence. Most of the applied voltage will appear acrossthe cathode dark space adjacent uneroded cathode sputter surface 13.Electric field intensifies in the cathode dark space region will be muchgreater than in the absence of the glow discharge. The electric fieldlines will, of course, be normal to uneroded cathode sputter surface 13,and hence will be generally transverse to magnetic field lines 95.

Energetic electrions are essential to sustaining the discharge by givingup their energy through a series of ionizing collisions with sputter gasatoms or molecules. Most of the energetic electrons which are capturedinto the discharge originate as a result of secondary electron emissionfrom the cathode sputter surface due to positive ion bombardment. Theseelectrons are immediately acted upon by the strong electric field in thecathode dark space, and are accelerated into the negative glow regionabove the cathode dark space. This leads to electron paths which aregenerally cycloidal in nature, with the electrons driftingcircumferentially above cathode 12 and around central anode 10. Thegenerally transverse nature of the electric and magnetic fields to eachother above uneroded cathode sputter surface 13 means that most of theenergetic electrons which are captured into the discharge will havevelocities which are predominantly perpendicular to rather than parallelwith the magnetic field lines, that is, υ.sub.∥ will generally besignificantly less than υ.sub.⊥. Thus these electrons can be confined bya magnetic mirror of modest strength, which is fortunate in view of thecrucial role played by these electrons in sustaining the discharge. Themain need for a strong magnetic mirror arises from discharge initiationconsiderations.

The magnetic field data shown in FIG. 3, exemplified by encircled datapoint 96, reveal that the magnetic field intensity along arepresentative magnetic field line 95 varies slowly from the point whereit exits uneroded cathode sputter surface 13 to the general center ofthe corresponding modified magnetic tunnel. Near the inner edge ofcathode 12, the magnetic field intensity along this magnetic field linehas approximately doubled. Thus the magnetic mirror used in the modifiedmagnetic tunnel of the preferred embodiment of the present invention, asshown in FIGS. 1 and 3, has a strength, as defined earlier, of about 2,which is sufficient to reflect back into the glow discharge mostelectrons for which υ.sub.∥ is less than υ.sub.⊥. The experimentalfindings, as described earlier, are that the new sputter source exhibitssuperior performance in most, if, indeed, not all, significant respectsover the prior art sputter source of FIGS. 2 and 4. These resultsconfirm the utility in magnetically enhanced sputter sources of magneticmirrors in which the mirror strength is on the order of 2.

The demonstrated utility of magnetic mirrors in sputter source providesa new dimension of design freedom. As shown in FIGS. 1 and 3, themagnetic circuit has been configured to produce magnetic field lineswhich are substantially parallel to uneroded cathode sputter surface 13over most of its extent. This is in sharp contrast to the prior artsputter sources in which the magnetic field lines exit and reenter thecathode sputter surface, forming relatively narrow arches over thecathode, as shown, for example, in FIGS. 2 and 4. Because the magneticfield lines in FIG. 3 are substantially parallel to uneroded cathodesputter surface 13, the glow discharge is more spread out and moreuniform in intensity than in prior art sputter sources employingconventional magnetic tunnels. Moreover, the intrinsic softness of themagnetic mirror allows the glow discharge to extend radially inwardfarther than would be the case for the electrostatic reflection whichdominates in conventional magnetic tunnels. Finally, the use of amagnetic mirror has made it feasible to reduce the change in magneticfield intensity averaged over the glow discharge as the cathode iseroded away by sputtering. These just-mentioned factors are in largemeasure responsible for the superior sputter source performancedescribed earlier.

Realization of a magnetic mirror is accomplished by so configuring themagnetic circuit that the magnetic field lines converge. Suppose, forexample, that field lines passing through transversely oriented area A₀in the central region of the glow discharge, and that these samemagnetic field lines pass through transversely oriented area A_(a)adjacent the anode. If B_(o) is the average intensity of the magneticfield lines passing through A_(o), then the average magnetic fieldintensity adjacent the anode, B_(a), is ##EQU4## and the strength of themagnetic mirror, as defined previously, is just magnetic mirror strength##EQU5## In the preferred embodiment of the present invention as shownin FIGS. 1 and 3, the magnetic field lines appear to converge by afactor of approximately 1.6 in the transverse plane of FIG. 3 in goingfrom the central region of the glow discharge to the inner edge ofcathode 12. In addition, these magnetic field lines converge further bythe radial convergence factor, which is the effective radial distance tothe central region of the glow discharge divided by the inner radius ofcathode 12. This radial convergence factor may be taken to be about 1.4,leading to a magnetic mirror strength of approximately 1.6×1.4=2.2.

Much of the discussion concerning the modified magnetic tunnels of thepresent invention has been restricted to the situation in which cathode12 is new, and hence uneroded. In this configuration, the side of themagnetic tunnel near the outer edge of cathode 12 is formed by magneticfield lines which pass through uneroded cathode sputter surface 13.These magnetic field lines do not reenter cathode 12, but instead forman arch to anode 10. Along the outer side of the magnetic tunnel,electrons are reflected electrostatically from cathode sputter surface13 back into the glow discharge. On the inner side of the magnetictunnel, electrons are reflected back into the discharge by magneticmirroring.

In normal operation the cathode is eroded away by sputtering. Afterabout 400 kilowatt-hours of normal operation with an aluminum cathode,the end-of-useful-life cathode sputter surface profile 113' has beenattained. With this eroded configuration, some of the magnetic fieldlines which exit cathode sputter surface 13' near the outer edge nowreenter this surface en route to anode 10. It is likely that byend-of-useful-life the discharge is confined by a magnetic tunnel inwhich electrons are reflected largely electrostatically along the innerside of the tunnel, as well as along the outer side, rather than beingreflected by magnetic mirroring as they were when the cathode was new.It is believed that this conversion from a soft magnetic mirror to ahard electrostatic mirror causes the glow discharge to become moreconcentrated in a narrower ring toward the outer edge of cathode 12,leading to comparatively rapid erosion in that region asend-of-useful-life approaches. Based on experimentally observedevolution of cathode sputter surface profiles with operation, it isbelieved that this change in the mode of reflection occurs near theend-of-useful-life cathode. As noted earlier, the glow dischargenormally extends several millimeters above the cathode. Hence, themagnetic field which is effective in confining the glow discharge is afield averaged in some fashion over the discharge rather than being justthe field averaged over the cathode sputter surface.

The fact that a transition from magnetic mirror reflection toelectrostatic reflection occurs reduces erosion near the inner edge ofcathode 12; it also hastens end-of-useful-life by concentrating thedischarge in a narrower ring toward the outer edge of cathode 12. Whileit would be preferable to avoid this change in mode of reflectionaltogether, the fact remains that greatly improved sputter sourceperformance is achieved with the preferred embodiment of the presentinvention, as shown in FIGS. 1 and 3, over prior art sputter sourcessuch as the one shown in FIGS. 2 and 4. As discussed earlier, this isdue in large measure to the new dimension of design freedom which theuse of magnetic mirrors allows.

While the invention has been described with reference to specificarrangements of parts, the description is illustrative of the inventionand is not to be constured as limiting the invention. Variousmodifications and applications may occur to those skilled in the artwithout departing from the true spirit and scope of the invention asdefined by the appended claims.

I claim:
 1. Glow discharge sputter coating apparatus comprising:cathodemeans including an annular sputter target having a surface from whichmaterial is to be sputtered and an inner and an outer rim eachintersecting said sputter surface; a magnetic system including magneticpoles, said poles comprising an inner pole adjacent said inner rim ofthe target, and an outer pole adjacent said outer rim of the target,said poles being of opposite magnetic polarity for producing a magneticfield comprising field lines which pass from the inner pole over theadjacent portion of said sputter surface without first intersecting saidsputter surface and then intersect said sputter surface toward the outerpole; said inner magnetic pole being electrically insulated from saidcathode means whereby said inner magnetic pole can be operated atpositive potential relative to said cathode means; and the configurationand location of said magnetic poles being such that the magnetic fieldintensity increases toward said inner pole from a region between saidpoles to form a magnetic mirror to confine electrons in the magneticfield adjacent said inner pole in an amount sufficient to provide saidglow discharge when said inner pole is at a positive potential withrespect to said cathode means.
 2. Glow discharge sputter coatingapparatus as claimed in claim 1 comprising means for operating saidinner pole at sufficiently positive potential to form an anode for saidapparatus.
 3. Glow discharge sputter coating apparatus as claimed inclaim 2 wherein said outer pole is electrically insulated from saidcathode means whereby said outer pole can be operated at positiveelectrical potential relative to said cathode means to avoid sputteringfrom the outer pole.
 4. Glow discharge sputter coating apparatus asclaimed in claim 1 wherein said sputter surface is in the form of aninverted generally conical surface.
 5. Glow discharge sputter coatingapparatus as claimed in claim 4 wherein said target has a bottom surfaceopposite said generally conical surface, said outer rim intersects saidbottom surface and said generally conical surface, said outer rim havinga lower portion intersecting said bottom surface and an upper portionintersecting said generally conical surface, said upper portion being oflarger diameter than said lower portion, and a cooling jacketsurrounding said lower portion and shielded by said upper portion. 6.Glow discharge sputter coating apparatus as claimed in claim 5 whereinsaid inner rim includes a recess having a clamping surface slopinginwardly and downwardly toward said bottom surface, and clamping meansfor engaging said clamping surface.
 7. Glow discharge sputter coatingapparatus as claimed in claim 4 wherein said outer pole is an annularmember surrounding said sputter target, and the top of said inner poleis below the top of said outer pole.
 8. Glow discharge sputter coatingapparatus as claimed in claim 1 wherein said annular sputter target hasa cross sectional shape wherein the inner rim is sloped outwardly fromits intersection with said sputter surface, said inner pole has an outeredge received within said inner rim of said target, and said outer edgeof said inner pole has a slope generally matching the slope of saidinner rim of said target.
 9. Glow discharge sputter coating apparatus asclaimed in claim 8 wherein said inner pole has a removable peripheralring portion adjacent said inner rim of said sputter target.
 10. Glowdischarge sputter coating apparatus as claimed in claim 1 wherein saidmagnetic system comprises a permanent magnet and an electromagnet coil,whereby the strength of the magnetic field can be adjusted by adjustingthe current flowing in said electromagnet coil.
 11. Glow dischargesputter coating apparatus as claimed in claim 1 wherein said magneticsystem includes centrally located magnet means concentric with saidinner pole, an outer cylinder of magnetic material magnetically joinedto said outer pole, and a plate of magnetic material forming a magneticcircuit between said magnet means and said outer cylinder.
 12. Glowdischarge sputter coating apparatus as claimed in claim 1 wherein themagnetic field intensity at said inner pole is greater than the magneticfield intensity at said region of the magnetic field by a factor of atleast about
 2. 13. Glow discharge apparatus as claimed in claim 1wherein said outer pole is electrically insulated from said cathodemeans and adapted to operate at the same potential as said inner pole.14. Glow discharge sputter coating apparatus comprising:cathode meansincluding an annular sputter target having a surface from which materialis to be sputtered and inner and outer rims each intersecting saidsurface; anode means positioned within the area surrounded by saidtarget with the outer perimeter of said anode means being locatedadjacent said inner rim of the target; said anode means being insulatedfrom said cathode means and being adapted to operate at positivepotential relative to said cathode means to provide the electrical fieldfor the glow discharge; a magnetic system comprising magnet means and anouter pole located adjacent said outer rim of the target, and said anodemeans comprising magnetic material forming an inner pole; said outer andinner poles being of opposite magnetic polarity for establishing amagnetic field over said sputter surface; and the configuration andlocation of said magnetic poles being such that the magnetic fieldintensity increases from a region between said poles to a positionadjacent said inner pole sufficiently to form a magnetic mirror toconfine electrons in the magnetic field at the inner pole side of themagnetic field in an amount sufficient to provide said glow discharge.15. Glow discharge sputter coating apparatus comprising:cathode meanscomprising an annular sputter target having a surface from which coatingmaterial is to be sputtered and inner and outer rims each intersectingsaid sputter surface; a magnet system including inner and outer magneticpoles positioned respectively adjacent said inner and outer rims, saidpoles being of opposite magnetic polarity for producing a magnetic fieldhaving field lines which pass between said poles over at least a portionof said sputter surface; said inner pole being electrically insulatedfrom said cathode means, whereby said inner pole is adapted to beoperated at a positive potential relative to said cathode means; theconfiguration and location of said magnetic poles being such that themagnetic field intensity increases toward said inner pole from a regionbetween said poles sufficiently to form a magnetic mirror to confineelectrons in the magnetic field adjacent said inner pole in an amountsufficient to provide said glow discharge when said inner pole is at apositive potential relative to said cathode means; and said inner polebeing positioned with respect to said target, such that said inner poleextends above the inward projection of a line interconnecting saidintersections of said inner and outer rims with said sputter surface.16. Glow discharge sputter coating apparatus comprising:an annular outershell of magnetic material; magnetic field forming means comprisingmagnet means centrally located within said shell and magneticallyconnected to said shell through means of magnetic material; annularcathode means substantially concentric with said shell and positionedinside said shell, said cathode means having radially inner and outerrims; an outer magnetic pole formed adjacent the end of said shellremote from said means of magnetic material and positioned adjacent saidouter rim; an inner magnetic pole adjacent said inner rim and ofopposite polarity from said outer pole, said inner pole being formed bysaid centrally located magnet means; and said inner pole beingelectrically insulated from said cathde means and adapted to form ananode for the glow discharge.
 17. Glow discharge apparatus as claimed inclaim 16 further comprising hermetic sealing means between said magnetmeans and said cathode means, whereby said magnet means can be atatmospheric pressure when said cathode means are in an evacuatedchamber.
 18. Glow discharge sputter coating apparatus employing magneticmirroring to confine electrons, said apparatus comprising:cathode meansincluding a sputter target having a surface from which material is to besputtered; a magnetic system including two magnetic poles of oppositepolarity positioned to form fringing magnetic field lines therebetween,at least some of said field lines having a central region passing oversaid sputter surface, and opposite end portions curving in the generaldirection of said sputter surface adjacent said two poles; said polesbeing positioned with respect to said sputter surface and each othersuch that at least some of the magnetic field lines pass from one poleover said sputter surface in a direction toward the other pole withoutfirst passing through said sputter surface; said one pole beingelectrically insulated from said cathode means and adapted to beoperated at positive electrical potential relative to said cathode meansto provide an anode for said sputter coating apparatus; and saidmagnetic system being such that the intensity of said magnetic fieldlines adjacent said one pole is substantially greater than in agenerally central region between said poles and by an amount which formsa magnetic mirror to confine electrons in the magnetic field adjacentsaid one pole in an amount sufficient to provide said glow dischargewhen said one pole is at positive potential relative to said cathodemeans.
 19. Glow discharge sputter coating apparatus as claimed in claim18 wherein the magnet field intensity adjacent said one pole is greaterthan the magnetic field intensity at said generally central portion ofthe magnetic field by a factor of at least about two.
 20. Glow discargesputter coating apparatus as claimed in claim 18 wherein both of saidpoles are electrically insulated from said cathode means.
 21. Glowdischarge sputter coating apparatus as claimed in claim 18 wherein thepoles of said magnetic system consist entirely of said two poles. 22.Glow discharge sputter coating apparatus as claimed in claim 18 whereinsaid magnetic field is such that after the sputter surface has beeneroded by ions in the glow discharge, some magnetic field lines passingfrom said one pole toward said other pole pass first through said targetand then exit from said eroded surface.
 23. Glow discharge sputtercoating apparatus comprising:annular cathode means adapted to beoperated at cathode potential; a magnetic system forming two magneticpole means of opposite priority for forming a magnetic field passingbetween said pole means; one of said pole means being positionedinwardly of said annular cathode means, and the other of said pole meansbeing positioned outwardly of said annular cathode means; the locationof said pole means relative to said cathode means being such that atleast some fringing magnetic field lines pass across said cathode meansin a closed loop annular configuration; and both said pole means beingelectrically insulated from said cathode means and adapted to beoperated at a positive electrical potential relative to said cathodemeans, whereby said pole means can provide an anode potential for saidglow discharge, and whereby sputtering from both of said pole means isprevented when said pole means are operated at positive electricalpotential relative to said cathode means.
 24. Glow discharge sputtercoating apparatus comprising:cathode means comprising an annular sputtertarget having a surface from which coating material is to be sputteredand inner and outer rims each intersecting said sputter surface; amagnet system including inner and outer magnetic poles positionedrespectively adjacent said inner and outer rims, said poles being ofopposite magnetic polarity for producing a magnetic field having fieldlines which pass between said poles over at least a portion of saidsputter surface;said inner pole being electrically insulated from saidcathode means, whereby said inner pole is adapted to be operated at apositive potential relative to said cathode means; said inner rim of thesputter target being sloped outwardly from its intersection with saidsputter surface; and said inner pole having a slope adjacent to andsubstantially matching said slope of said inner rim of the sputtertarget.